Idea sounds brilliant. Challenge is how much can be drawn from WiFi signal that is intended for other purpose. Would it make more sense to draw power from other means such as solar or thermo? On the other hands, for indoor use, I am not too sure energy harvesting is crucial to the future of IoT. I can see outdoor IoT definitely demands energy harvesting though.

" By culling energy from existing radio, TV, and wireless signals, they've developed a way to connect low-energy devices, such as sensors and wearables, to the Internet without the need for batteries or power cords."

Hey! I just mentioned that idea a week ago in a comment! How did they develop it so fast?

Just kidding--obviously they came up with idea before me. This would be great for stress sensors on bridges and buildings. They are very low-power and low speed, so they need almost no power. They could slowly build up enough energy to be able to send out their data when needed.

This idea of harvesting energy is very good for sensor netwroks, where the sensors are so remotely placed that distributing power to them via conventional power netwroks is expensive and batteries are not feasible because of the maintenance and recharge issues.

Today, our atmosphere is filled with all kind of RF signals - internet traffic on wi-FI routers, FM radio signals, TV signal traffic over various frequency bands and so on.

So a good and judicious harvesting of these signals to generate the necessary power of sensors that are in open atmosphere and away from the power distributions networks will be defintely beneficial.

"Scavenging experiments utilizing TV bands have already yielded power amounting to hundreds of microwatts, and multi-band systems are expected to generate one milliwatt or more. That amount of power is enough to operate many small electronic devices, including a variety of sensors and microprocessors."

There are also some interesting projects in Kickstarter, such as iFind/Wetag, though I don't understand their tricks clearly.